[C++] Add kernels for comparison operations to scalars

This should implement the comparison operators >=, >, ==, !=, <, <= between arrow::compute::Datum and arrow::compute::Scalar.

The result of this kernel will be a boolean type arrow::compute::Datum where with True/False set according to the outcome of the operation and NA if a row was not valid.

A pre-condition to implement this kernel is to have a working implementation of arrow::compute::Scalar.